1. Field of Art
This invention relates to electrically tunable surface acoustic wave variable delay lines, and more particularly to control of surface acoustic wave velocity by variation of the carrier concentration in the bulk of a piezoelectric and semiconductive SAW module.
2. Description of the Prior Art
As is known, the frequency of a surface acoustic wave (SAW) propagating in a piezoelectric medium is a faithful reproduction of the electrical wave applied to an electroacoustic transducer on the surface of the SAW module. The velocity of the wave propagating in the module is, on the other hand, dependent upon the material itself. And the wavelength or periodicity of such a wave is a combined function of the frequency and the inherent wave velocity of the material. Surface acoustic wave delay lines are used as frequency determining elements in circuits, such as ocillator loops including a SAW device and an amplifier. In some cases, the SAW device is utilized because it has a property that alters the velocity of the SAW wave as a consequence of mechanical stress of the SAW module, such as in force and pressure transducers. In other cases, the variation in SAW velocity as a function of temperature is utilized.
In the more general case, SAW devices are used as principal frequency control elements for oscillators, due to the capability of controlling the SAW module so that its parameters remain essentially constant within a desired tolerance, thereby controlling the frequency of an oscillator loop in which it is included to a desired degree of accuracy. Important advantages in utilizing SAW devices in comparison, for instance, with conventional bulk acoustic resonators, are that the fundamental frequencies may be higher by an order of magnitude in the SAW devices, and the planar fabrication techniques for SAW devices promote low cost batch production.
The simplest SAW frequency control element is simply a delay line. Another typical SAW frequency control element is a resonator having reflector contacts or surface discontinuities extending from either side of the launch and detector transducers which are separated by an interaction region. If the inherent wave propagation velocity of the material is kept constant, the least lossy SAW propagation will occur at a frequency which has, at that constant SAW velocity, a wavelength which is in phase for all successive waves reflected from the reflection elements of the resonator, thereby defining a preferred frequency for control of an oscillator loop formed by connection in series with an amplifier. The most common form of frequency control used with SAW delay lines and resonators of the prior art is an external, variable phase shifting circuit in series with the SAW element. This allows fine-tuning of a closed-loop oscillator frequency, to a limited extent. As is known in the art, the velocity of a surface acoustic wave in a highly piezoelectric medium, such as lithium niobate, is slower when the surface of the medium is electrically short circuited (such as by a metallic conductor) then it is when the surface is unshorted. The difference in the two velocities (.DELTA.V) divided by the unshorted velocity (V) provides an indication (.DELTA.V/V) of the variation in frequency of an oscillator loop (.DELTA.f/f) incorporating such a medium. This phenomenon is used to advantage to control the resonant frequency of a reflective resonator, of the type referred to briefly hereinbefore, in a device described in Cross, P. S. et al, Electronically Variable Surface-Acoustic-Wave Velocity and Tunable SAW Resonators, Applied Physics Letters, Vol. 28, No. 1, Jan. 1976, pp. 1-3. Because of the fact that the .DELTA.V/V obtainable with highly piezoelectric material such as lithium niobate is quite high, only a small portion of the SAW propagation path need be subjected to selective short circuiting in order to provide a usefully high .DELTA.f/f, which is on the order of 1.4%. However, in order to provide such a module in an oscillator configuration integrally formed therewith in a monolithic fashion, semiconductive properties are required. Active circuits are inherently incapable of being formed monolithically into integrated circuit structures, such as SAW controlled oscillators, utilizing material which is only piezoelectric, such as lithium niobate. On the other hand, the .DELTA.V/V obtainable with surface shorting of piezoelectric semiconductive materials (which can support active and passive device fabrication for integrated circuits) is small, being several orders of magnitude below that for lithium niobate.
A variable delay line employing a piezoelectric and semiconductor substrate in which carrier concentration is controlled from the surface so as to alter the velocity of surface acoustic waves within the substrate is described in Chao, G., Monolithic Surface-Acoustic-Wave Phase Shifter, Electronics Letters, Feb. 8, 1973. Therein, a Schottky-barrier distributed varactor diode is fabricated on a cadmium sulphide substrate. The diode consists of a thin film of gold distributed across the entire interaction region between input and output transducers. However, this device provides less .DELTA.V/V than might be expected since it inherently has a surface shorting characteristic caused by the diode metallization.